Red means "stop," green means "go," and yellow means "hurry up and make that damn light." Why those colors, though? Why not blue, purple, and brown? I have to admit that aside from a hunch that it had to do with wavelengths, I had no idea myself, so I decided to look into it.
紅燈“停”，綠燈“行”，黃燈亮了“快點走”，但為什么交通信號燈要用這三個顏色，而不用藍色、紫色或者褐色呢？必須承認除了直覺認為這與波長有關外，我完全不知道原因，所以我決定一探究竟。

The answer, as it turns out, is a little convoluted, but makes sense. The earliest traffic signals were designed for trains, not cars. They were red and green, gas-powered, and more than a little dangerous in the event of a leak.
事實證明，答案確實有點復雜，不過倒也還講得通。最早的交通信號燈是為火車而不是汽車通行設計。信號燈為紅色和綠色，使用煤氣，在漏油時會非常危險。

? Red is an inherited symbol from railroads
紅色源于鐵路信號燈

Red symbolizes danger in many cultures, which makes sense, considering it has the longest wavelength of any color on the visible spectrum, meaning you can see it from a greater distance than other colors. Red has meant stop since long before cars existed, with train signals' use of red dating back to the days when mechanical arms lifted and lowered to indicate whether the rail ahead was clear. So that one's simple.
由于紅色在可見光譜中波長最長，因此可視距離要比其它顏色更遠，這就可以理解為什么很多文化中將紅色視為危險信號。紅色很早以前就被用作火車信號，那時汽車還沒出現，而鐵路用機械臂板上升下降來代表前方是否有火車駛來。這比較容易理解。

? Green meant "caution" at first
綠色最早用于“警示”

Green's role in lights has actually changed dramatically over time. Its wavelength is next to (and shorter than) yellow's on the visible spectrum, meaning it's still easier to see than any color other than red and yellow. Back in the early days of railway lights, green originally meant "caution," while the "all-clear" light was, well, clear or white. Trains, of course, take an interminably long time to stop, and legend has it that several disastrous collisions happened after an engineer mistook stars in the night horizon for an all-clear. Thus, green became "go," and for a long time, railways used only green and red to signal trains.
長期以來綠色在交通信號燈中的角色發生了巨大變化。可見光譜中，綠色波長因僅次于黃色，因此是除了紅色和黃色外最容易看見的顏色。在早期的鐵路信號燈中，綠色最初代表“警示”，而無色或白色代表“全線通行”。當然，火車停下來需要非常長的時間，據說由于一名工程師將夜晚天空中的星星誤認為全線通行的信號燈，導致了若干起災難性事故的發生。因此，綠燈成為了“通行”的標志，長期以來鐵路也只將紅色和綠色作為信號燈。

? Yellow means "caution" because it's almost as easy to see as red
黃色代表“注意”，因為它幾乎和紅色一樣顯眼。

From the earliest days of motoring up until the mid-1900s, not all stop signs were red --many were yellow, along with yield signs, because at night it was all but impossible to see a red stop sign in a poorly lit area. The yellow stop-sign craze began in Detroit in 1915, a city that five years later installed its first electric traffic signal, which happened to include the very first amber traffic light, at the corner of Michigan and Woodward Aves.
從最早開始出現汽車到20世紀中期，并不是所有的停車信號都用紅色——很多使用黃色，讓路標志也是，因為夜晚在光線極暗的地方根本看不見紅色的停車信號。“黃色停車信號潮”始于1915年的底特律，五年后，這座城市在密歇根和伍德沃德大街安裝了第一個首次包括黃色燈的電子交通信號燈。

But what of those weird yellow stop signs, you ask? As materials and technologies evolved, the ability to produce highly reflective signs meant that red could resume its natural spot in the sign hierarchy, leaving the still-highly-visible yellow (it's second only to red in terms of visible wavelength) to the domain of "caution." That's why school zones and buses, crosswalks, and other important warnings are yellow today.
你是不是想問，那些奇怪的黃色停止標志又是干什么的呢？材料學和高技術的發展使得人們研發出高度反光指示燈，因此紅色才能在信號燈中重拾“停車霸權”，將可視度較高的黃色（可視波長僅次于紅色）留在了“警告”的位置。這也是為什么學校區、公交站、人行橫道和其它重要位置使用黃色警告信號燈的原因。

英文來源：thrillist.com
翻譯：張金瑤（中國日報網愛新聞iNews譯者）
編審：yaning